First Alert Plug-In CO and Combo Smoke Alarms, Explained

Carbon monoxide, or CO, is a colorless, odorless gas that can come from fuel-burning appliances such as gas furnaces, water heaters, gas dryers, fireplaces and space heaters, as the SC9120FF listing explains. You cannot see or smell it, so a CO alarm uses a chemical sensor, usually electrochemical, to measure it in the air. Smoke from a fire is detected differently, using a sensor that notices particles in the air. Because they work so differently, a CO-only alarm will not warn you about a fire, and a smoke-only alarm will not warn you about CO. A combination alarm has both sensors in one case.

High score Our score: 9.2/10

First Alert

FIRST ALERT CO605 Plug-in Carbon Monoxide Detector with Battery Backup, White

9.2/10Our score, based on listed specifications and buyer feedback

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Price and specifications are read from the Amazon product page and can change.

FIRST ALERT CO605 Plug-in Carbon Monoxide

Best for: Renters wanting plug-in CO with backup

Pros

• Battery backup in outages
• Electrochemical CO sensor
• 7-year limited warranty

Cons

• Detects CO only, not smoke
• Model number varies in listing

The FIRST ALERT CO605 Plug-in Carbon Monoxide Detector plugs straight into any standard outlet, with no mounting, and has a battery backup so it keeps monitoring during a power outage. It uses an electrochemical CO sensor and sounds a loud 85-decibel alarm when CO reaches dangerous levels. The listing says it detects accurately no matter where it is placed in the room.

A single button handles testing and silencing, and a low-battery mute quiets the battery chirp for up to 8 hours. An end-of-life chirp tells you when the unit needs replacing, and it carries a 7-year limited warranty. The listing says it meets UL standards and measures 3.4 by 1.38 by 5 inches. Note that the listing mixes model names: the title says CO605, while the specifications and description refer to CO606.

This suits renters and anyone who wants CO protection without drilling holes, especially near bedrooms. It does not detect smoke, so pair it with a smoke alarm or choose a combo unit.

High score Our score: 9.8/10

First Alert

First Alert Plug in Carbon Monoxide Alarm

9.8/10Our score, based on listed specifications and buyer feedback

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Price and specifications are read from the Amazon product page and can change.

First Alert Plug in Carbon Monoxide

Best for: Simple plug-in CO for bedrooms

Pros

• Plugs into standard outlet
• 85-decibel alarm
• End-of-life timer

Cons

• No battery backup
• Detects CO only

The First Alert Plug in Carbon Monoxide Alarm, model CO600, is the simplest option here. It plugs into a 120-volt wall outlet, uses an electrochemical CO sensor and sounds an 85-decibel alarm when CO levels rise. One button tests or silences it, and an end-of-life timer signals when to replace it. It has a 7-year limited warranty.

The key difference from the CO605 is that the CO600 has no battery backup and no display, as the listing states. If the power goes out, or the plug is pulled, it stops protecting you. That matters because some CO risks, such as running a generator or a gas heater, happen during outages.

This suits a bedroom or hallway where the outlet is reliable and you want a basic CO alarm. For outage protection, the CO605 is the better pick. Avoid plugging any safety alarm into a smart plug that could switch it off by mistake.

First Alert

First Alert Hardwire Combination Smoke & Carbon Monoxide Alarm

8.4/10Our score, based on listed specifications and buyer feedback

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Price and specifications are read from the Amazon product page and can change.

First Alert Hardwire Combination Smoke &

Best for: Replacing wired alarms across a home

Pros

• Smoke and CO in one
• Interconnects with compatible alarms
• Precision Detection for fewer cooking false alarms

Cons

• Needs existing hardwired setup
• Not a wall-outlet plug-in

The First Alert Hardwire Combination Smoke & Carbon Monoxide Alarm, model SMICO100-AC, is a 2-in-1 alarm that wires into your home’s electrical system. The listing shows a unit count of 3, which suggests a multi-pack, so check the quantity on the page. It uses First Alert’s Precision Detection technology, which the listing says meets new industry standards for reducing cooking nuisance alarms while giving early fire warning.

It interconnects with compatible alarms, so when one sounds, they all sound. A Quick Connect Plug and Easy Adapter Plug are listed for installing without rewiring, which usually means replacing existing hardwired alarms. A 9V battery backup keeps it working during outages, and a test/silence button is included.

This suits homeowners who already have hardwired alarms and want to upgrade to smoke and CO together. It does not plug into a wall outlet, so renters without wiring should look at the SMCO210.

First Alert

First Alert 10-Year Battery Combination Smoke & Carbon Monoxide Alarm

9.0/10Our score, based on listed specifications and buyer feedback

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Price and specifications are read from the Amazon product page and can change.

First Alert 10-Year Battery Combination Smoke

Best for: Renters and rooms without wiring

Pros

• 10-year sealed battery
• Smoke and CO in one
• Slim, half-depth profile

Cons

• Not interconnected in listing
• Whole unit replaced at end of life

The First Alert 10-Year Battery Combination Smoke & Carbon Monoxide Alarm, model SMCO210, gives 2-in-1 protection with no wires and no outlet. Its 10-year sealed battery means no battery changes and no low-battery chirps for the life of the alarm. It also uses Precision Detection technology to cut down on cooking nuisance alarms.

It has a slim profile about half the depth of a standard alarm, and an end-of-life warning tells you when to replace the whole unit. The listing does not mention interconnection with other alarms.

This is the closest match for someone who wanted a “plug-in” combo: it installs anywhere without wiring and covers both smoke and CO. For linked alarms throughout a house, the hardwired models are the choice.

Best for: Hardwired homes needing combo replacements

Pros

• Smoke and CO detection
• Interconnects with BRK and First Alert
• 10-year limited warranty

Cons

• Ionization smoke sensor only
• Requires hardwiring

The FIRST ALERT BRK SC9120FF Hardwired Smoke and Carbon Monoxide Detector combines an electrochemical CO sensor with an ionization smoke sensor. Ionization sensors are generally quick to react to fast, flaming fires. Indicator lights on the face show whether smoke or CO triggered the alarm, and an 85-decibel siren gives the warning.

It connects to other compatible BRK or First Alert alarms so all sound together. A 9V battery backup, side-load battery door, single test/silence button, tamper-resistant locking brackets and universal mounting brackets are listed, along with a 10-year limited warranty and UL testing. The specifications refer to model SC9120B while the title says SC9120FF.

This suits a home with existing BRK or First Alert hardwired alarms. Buyers who want the newer Precision Detection technology may prefer the SMICO100-AC.